1. Field of the Invention
The present invention relates to an optical module.
2. Prior Art
In order to realize a multimedia-based society, it is indispensable to set up an effective infrastructural arrangement for multimedia communication systems. Among other various plans for realizing that, recently, xe2x80x9cThe Fiber To The Home (FTTH)xe2x80x9d plan has been pushed forward attracting a great deal of attention. This is the plan according to which respective personal homes are connected with one another by means of optical fiber cables extended thereto, thereby establishing a higher speed digital communication network among them. In this case, it is necessary for such an optical communication system using optical fiber cables as mentioned above to be provided with an optical transmitter module having an ability to convert an electrical signal into an optical signal and then output the converted optical signal to the optical fiber cable, and an optical receiver module having an ability to receive the optical signal transmitted through the optical fiber cable and then convert it into an electrical signal, or to be provided with an optical transmitter-receiver module having both of these functions.
In general, the optical transmitter-receiver module includes optical waveguide elements, optical fiber cables and optoelectronic parts, and the optical fiber cables and optoelectronic parts are to be coupled with the optical waveguide. These optical waveguide elements, optical fiber cables and optoelectronic parts are integrally mounted on a silicon (Si) bench substrate.
In a prior art optical transmitter-receiver module, as disclosed in the JP unexamined Patent Publication (KOKAI) No. H10-133069 (Title: Optical Transmitter-Receiver, Assignee: Kyosera K. K.), the optical waveguide element is mounted on an Si bench substrate such that the upper surface of an optical waveguide layer forming the optical waveguide element directly lies on the upper surface of the Si bench substrate. The film thickness of a clad layer forming the optical waveguide layer has been controlled such that the height (optical axis height) of a core layer forming the optical waveguide layer, from the upper surface of the Si bench substrate coincides with the optical axis height of the optical fiber cable and also with that of the optoelectronic part.
In general, the optical waveguide layer (core layer, clad layer) of the optical waveguide element made of the quartz having the excellent optical property is formed by means of the Flame Hydrolysis Deposition (FHD) method used for manufacturing optical fiber cables or the Chemical Vapor Deposition (CVD) method used for manufacturing semiconductor elements. When forming the optical waveguide layer by one of these methods, however, there is possibility of causing the problems as described in the following.
According to the FHD method, a raw material is first hydrolyzed in the oxyhydrogen flame, thereby heaping up fine particles of glass. Then, these fine glass particles are heated at a high temperature of more than one thousand several hundreds degrees centigrade, thereby producing a transparent glass layer. With this heat treatment, fine glass particles are melted and firmly combined together to form a dense glass layer (i.e., optical waveguide layer). However, owing to not only the large cubical contraction coefficient of the glass layer formed by FHD method but also difficulty in controlling the film thickness of this glass layer, the mean value of the film thickness is apt to indicate an undesirable large fluctuation and to make the tolerance larger, accordingly. Under certain circumstances, there is possibility of experiencing variation of about 20% with respect to the glass layer film thickness. For instance, if the optical waveguide element is mounted on the Si bench substrate under the condition that the optical axis height of the core layer is set to 10 xcexcm, optical axis misalignment of plus or minus 2 xcexcm may be caused in the optical waveguide element, so that the optical coupling loss between the optical waveguide element and the optical fiber cable or the optoelectronic part might be increased to the extent that information transmission might be adversely affected to invite deterioration thereof.
According to the CVD method, since the glass layer is formed through the process of CVD reaction, there is no need to carry out such a heat treatment as described above. Consequently, the film thickness of the glass layer can be controlled with ease, and the film thickness can be managed within a range of at most 5% of a set value even though it is fluctuated. For instance, if the optical waveguide element is mounted on the Si bench substrate under the condition that the optical axis height of the core layer is set to 10 xcexcm, the optical axis misalignment in the optical waveguide element can be managed within the range of plus or minus 0.5 xcexcm. Therefore, the optical coupling loss between the optical waveguide element and the optical fiber cable or the optoelectronic part might become so small that it is possible to neglect that loss.
However, in the process of manufacturing the optical waveguide element by heaping up glass layers in a chamber for carrying out the CVD method, some fine glass particles once deposited on the inside wall surface of the chamber come off therefrom, fall down on the top surface of the glass layers being processed, and are fixed thereto. Consequently, the falling fine glass particle causes a defect in the form of a projection resulting in an uneven surface of the glass layer. This projection shaped defect in the glass layer further gives the variation of several microns to the optical axis height of the optical waveguide element, so that it might cause an increase in the coupling loss between the optical waveguide element and the optical fiber cable or the optoelectronic part. Because of the structure of the CVD equipment, it would be hard to completely prevent that during the CVD process, and the fine glass particle is deposited on and then comes off from the inside wall of the CVD chamber, Therefore, it would be actually impossible to completely protect the glass layer from the defect in the form of the projection as mentioned above.
As has been discussed above, according to the prior art optical transmitter-receiver module, if the film thickness of the optical waveguide layer is not uniformly formed or the projection shaped defect is caused on the surface of the optical waveguide, the optical axis height of the waveguide element is varied. Therefore, it is difficult to establish the excellent optical coupling between the optical waveguide element and the optical fiber cable or the optoelectronic part.
Accordingly, the invention has been made in view of problems as described above, and its main object is to provide an optical module in which the optical axis of the optical waveguide element is made adjustable with high accuracy.
In order to solve the problems as described above, the invention provides an optical module which includes an optical waveguide element having an optical waveguide, and a bench substrate on which the optical waveguide element is mounted. The optical waveguide element is mounted on the bench substrate through one or more spacers. According to the optical module having the constitution described above, the optical axis height of the optical waveguide element, that is, the height from the upper surface of the bench substrate to the optical waveguide can be adjusted with high accuracy by regulating the size of the spacer.
Each of the spacers is preferably prepared in the form of a sphere. The spherical spacer is advantageous. For instance, when mounting the optical waveguide element on the bench substrate, if the optical waveguide element includes some defects such as projections on its surface directly facing the bench substrate, each spherical spacer having met with the projection can rotate itself to seek and move to a more flat place, thereby avoiding meeting with and staying on the projection. Thus, the gap distance between the optical waveguide element and the bench substrate can be regulated with the diameter of the spherical spacer. Furthermore, it is preferable to use the spherical spacer which is made of a material such as quartz having a small thermal expansion coefficient. With this, even if the ambient temperature surrounding the optical module is changed, it is possible to keep the height from the upper surface of the bench substrate to the optical waveguide constant.
The bench substrate has one or more first recesses on its surface on which the optical waveguide element is mounted. Each of the spherical spacers is accepted in part by these first recesses. Consequently, when the optical waveguide element is mounted on the bench substrate, each spherical spacer can be prevented from being scattered.
The optical waveguide element has one or more second recesses on its surface which faces the bench substrate when the optical waveguide element is mounted on the bench substrate. Each of the spherical spacers is accepted in part by these second recesses, so that the gap distance between the bench substrate and the optical waveguide element is regulated by the spacer. At the same time, the optical waveguide element is positioned such that it directs to a predetermined direction with respect to the bench substrate surface.
If each of the first and second recesses are respectively prepared in the form of a V-shaped groove, the position (one axis) of the optical waveguide element in the direction intersecting the longitudinal axis of the V-shaped groove at right angles can be determined.
Each of the first recesses is prepared in the form of a first cavity which can accept one of the spacers. This first cavity is preferably prepared in such a shape that it has inside wall surfaces with which the surface of each spacer accepted therein can make contact at least at three points. Also, each of the second recesses is prepared in the form of a second cavity which can also accept one of the spacers. This second cavity is preferably prepared in such a shape that it has inside wall surfaces with which the surface of each spacer accepted therein can make contact at least at three points. According to the constitution like the above, the position (two axes) of the optical waveguide element can be determined with respect to the horizontal direction. Actually, these first and second cavities may be prepared in the form of an almost right pyramid or an almost circular cone. Furthermore, these cavities may be prepared in the form of a right pyramid or a circular cone having uneven portions on their inside wall surfaces.
The invention will now be described in detail with regard to several preferred embodiments of the optical module according to the invention, with reference to the accompanying drawings. In the following description and drawings, the constituents of the invention having like functions and constitutions are denoted with like reference numerals or marks in order to avoid repetitive description.